D-tagatose is the keto-sugar corresponding to the aldo-sugar D-galactose. It has a sweetness value equivalent to sucrose but is poorly digested and has been found to be a useful, safe non-cariogenic low-calorie sweetener in food products, for which there is high demand.
D-tagatose can be synthesised chemically, e g. as described in U.S. Pat. No. 5,002,612
Enzymatic methods for production of D-tagatose have been described. Yamanaka and Wood (1966) list a number of lactic acid bacteria providing an L-arabinose isomerase enzyme capable of producing ketoses from L-arabinose, D-galactose or D-fucose.
U.S. Pat. No. 6,057,135 discloses a process for manufacturing D-tagatose, wherein a lactose permeate is hydrolysed to obtain a lactose hydrolysate comprising D-galactose and glucose. The hydrolysate is fermented to convert the glucose to ethanol which is subsequently removed and the remaining solution of D-galactose is subjected to enzymatic isomerisation with an L-arabinose isomerase to obtain D-tagatose. The L-arabinose isomerase preparations used are crude biomass extracts of Lactobacillus pentosus, Bacillus amyloliquefaciens or Arthrobacter spp
WO 00/68397 describes the use of E. coli engineered for enhanced expression of E. coli L-arabinose isomerase for the production of tagatose.
WO 02/50282 describes the isolation of a thermostable L-arabinose isomerase capable of isomerising galactose. The amino acid sequence of this enzyme is closely related to previously known L-arabinose isomerase sequences, especially Bacillus stearothermophilus. 
However, to date, enzymatic methods for production of tagatose have not been used commercially. There exists a high demand for new and improved low-calorie sweeteners, and consequently, improved methods for producing tagatose with higher efficiency and yield are highly needed in the industry.
A novel L-arabinose isomerase active enzyme has now been isolated and characterised. This enzyme exhibits a low sequence similarity when compared to all presently known L-arabinose isomerase sequences, including those disclosed in WO 00/68397 and WO 02/50282. The enzyme of the current invention has different substrate specificity as compared to prior art L-arabinose isomerases and it is a versatile alone isomerase capable of isomerising structurally related aldoses The enzyme is obtainable from a thermophilic microbial source and can thus be used at high operating temperatures.